59 
OX THE MEA811REMEXT OF .MA(4NETIC HYSTERESIS. 
have rendered the coil ineffective in “choking” any sudden variation of the current 
due to a sudden change of E. M. F. or of resistance. The iron, which was supplied by 
Messrs. Crompton, was tested for magnetic quality with the following results :— 
Ho 
•182 
•371 
•570 
•854 
1-09 
1-53 
2-10 
O . o 
O Oi 
4-98 
6-90 
8-58 
11-15 
16-20 
Bo 
35-4 
103 
190 
390 
G60 
1510 
3090 
5260 
7300 
8560 
9470 
10200 
11500 
194 
278 
334 
457 
606 
987 
1470 
1560 
1470 
1240 
1100 
914 
710 
In magnetic quality, the iron is very nearly the same as the soft thin sheet iron 
(Pting Y.) tested by Professor Ewing and Miss Klaassen.^ 
The core was wound with three independent layers of cotton-covered copper wire 
as follows 
Layer. 
B.W.G. 
Turns. 
Ohms. 
1 
Magnetic force per unit 
C.G.S. current. 
1 
Xo. 15. 
225 
• 475 
44-5 , 
2 
Xo. 15. 
205 
•50 
40-4 
3 
No. 18. 
1 
265 
1-44 
52-3 
i 
Stout wire was used in order to avoid any considerable increase in the resistance of 
the primary circuit. This resistance must be kept low if the choking action is to be 
efficient. 
§ 34. This choking coil is perhaps unnecessarily large. It might be better to use 
a core smaller both in diameter and cross-section and to form each of the coils on it 
with wire of the same gauge as that used for a corresponding coil on the solenoid. By 
this expedient, if we alvvays employ corresponding coils on the choking coil and the 
solenoid, the magnetic force in the choking coil has the same value as in the solenoid. 
Thus, if the iron used in the choking coil is of approximately the same quality as the 
specimen under test, the coil will most effectively choke the primary current approxi¬ 
mately at the time when the choking is most needed, viz., when the value of cZB/dH 
for the specimen has its greatest value, as was explained in § 10. If the iron plates 
used in the core of the choking coil are of good quality it is very unlikely that any 
specimen will be so “ soft ” in the magnetic sense as to have a value of cZB/cZH so 
many times greater than the value of cZB/t/H for the core as to lead to inaccurate 
measurements. If the specimen is of “ hard ” iron or steel the value of cZB/cZH for 
the specunen is by comparison so small that it is of little consequence if the most 
efficient choking action takes })lace when dB/t/H for the specimen is not at its 
“Magnetic Qualities of Iron,” ‘Phil. Trans.,’ A, vol. 184 (1893), p. 1003. 
